Explanatory gaps

The thesis of supervenience has called attention to a particularly striking difficulty about how to integrate talk about minds into a general scientific understanding of the world, a difficulty that arises both in the case of conscious states and in the case of intentional ones. Although mental properties may well supervene on physical properties, it is surprisingly difficult to say exactly how they might do so.

Consider how most ordinary nonmental phenomena are explained. It is one of the impressive achievements of modern science that it seems to afford in principle quite illuminating explanations of almost every nonmental phenomenon one can think of. For example, most adults who want to understand why water expands when it freezes, why the Sun shines, why the continents move, or why fetuses grow can easily imagine at least the bare outlines of a scientific explanation. The explanation would consider the physical properties of trillions of little particles, their spatial and temporal relations, and the physical (e.g., gravitational and electrical) forces between them. If these particles exist in these relations and are subject to these forces, it follows that water expands, the Sun shines, and so on. Indeed, if one knew these physical facts, one would see in each case that these phenomena must happen as they do. As the American philosopher Joseph Levine nicely put it, the microphysical phenomena “upwardly necessitate” the macrophysical phenomena: water could not but expand when it freezes, given the properties of its physical parts.

But it is precisely this upward necessitation that seems very difficult to even imagine in the case of the mental, particularly in the case of the two phenomena discussed above—consciousness and intentionality. The easiest way to see this is to consider a simple puzzle called the “inverted spectrum.” How is it possible to determine whether two people’s colour experiences are the same? Or, to put the question in terms of physicalism: what physical facts about a person determine that he must be having red experiences and not green ones when he looks at ordinary blood? This problem is made especially acute by the fact that the three-dimensional colour solid (in which every hue, saturation, and tone of every colour can be assigned a specific location) is almost perfectly symmetrical: the reds occupy positions on one side of the solid that are nearly symmetrical with the positions occupied by the greens. This suggests that with a little tinkering—e.g., secretly implanting colour-reversal lenses in a child at birth—one could produce someone who used colour vocabulary just as other people do but had experiences that were exactly the reverse of theirs. Or would they be? Perhaps the effect of the tinkering would be to ensure not that the person’s experiences were the reverse of others’ experiences but that they were the same.

The problem is that it seems impossible to imagine how one could discover which description is correct. Unlike the case of the expansion of water, knowing the microphysical facts does not seem to be enough. One would like somehow to get inside other people’s minds, in something like the way each person seems to be able do in his own case. But mere access to the physical facts about other people’s brains does not enable one to do this. (An analogous problem about intentionality was raised by Quine: What physical facts about someone’s brain would determine that he is thinking about a rabbit as opposed to “rabbithood” or “undetached rabbit parts”?)

Indeed, to press the point further, it is not even clear how physical facts about a person’s brain determine that he is having any experiences at all. Many philosophers think that it is perfectly coherent to imagine that all of the people one encounters are actually “zombies” who behave and perhaps even think in the manner of a computer but do not have any conscious mental states. This is a contemporary version of the traditional problem of other minds, the problem of identifying what reasons anyone could have for believing that anyone else has a mental life; it is also sometimes called the problem of “absent qualia.” Again, the question to be asked is: What is it about the physical constitution of a creature’s brain that compels one to think that it has a mental life, in the same way that the physics of water compels one to think that it must expand when it freezes?

Substance dualism and property dualism

Confronted with the problems about identity and explanatory gaps, some philosophers have opted for one version or another of mind-body dualism, the view that mental phenomena cannot in any way be reduced to physical phenomena. In its most radical form, proposed by Descartes and consequently called Cartesianism, dualism is committed to the view that mind constitutes a fundamentally different substance, one whose functioning cannot be entirely explained by reference to physical phenomena alone. Descartes went so far as to claim (in accordance with contemporary church doctrine) that this substance was an immortal soul that survived the dissolution of the body. There are, however, much more modest forms of dualism—most notably those concerned with mental properties (and sometimes states and events)—that need not involve any commitment to the persistence of mental life after death.

Causal relations and epiphenomenalism

It is important to distinguish such claims about the dualistic nature of mental phenomena from claims about their causal relations. In Descartes’s view, mental phenomena, despite their immateriality, can be both causes and effects of physical phenomena (“dualistic interactionism”). The dualist does not ipso facto deny that physical phenomena in the brain quite regularly cause events in the mind and vice versa; he merely denies that those phenomena are identical to anything physical.

A problem with dualistic interactionism, however, concerns the evident lack of any causal break in the internal processes of the human body. So far as is known, there is no particular state of any part of the body—no action of any muscle, no secretion of any substance, no change in any cell—that cannot in principle be explained by existing physical theories, assuming it can be explained at all (quantum indeterminacy is irrelevant to the present point). Serious evidence of so-called “paranormal” phenomena, such as telepathy, is yet to be found. More generally, there seems to be very good reason to think that the physical world forms a closed system, obeying conservation laws such as the conservation of mass and the conservation of energy. Consequently, there would appear to be no explanatory need to introduce nonphysical phenomena, whether substances or properties, into any account of human activities. (In contrast, before the introduction of electromagnetism in the late 19th century, there were myriad phenomena that could not be explained without supposing the existence of another force in addition to gravitation.)

In response to this difficulty, dualists have tried to exempt the mental from any causal role. Leibniz claimed that mental events were neither causes nor effects of any physical events—they were simply “synchronized” by God with physical phenomena, a view known as “parallelism.” A more moderate position, originally advocated by the English biologist T.H. Huxley (1825–95) and revived by the Australian philosopher Frank Jackson in the late 20th century, is that mental phenomena are the effects, but not the causes, of physical phenomena. Known as “epiphenomenalism,” this view allows for the evident causal laws relating physical stimuli and perceptual experiences but does not commit the dualist to claims that might conflict with the closure of physics.

These responses, however, may serve only to make the problem worse. If the mental really does not have any effects, then it becomes entirely unclear why one should believe that it exists. What possible reason could there be for believing in the existence of something in the spatiotemporal world that does not affect anything in that world in any way? Epiphenomenal mental phenomena would seem to be no different in this respect from epiphenomenal angels who accompany the planets without actually pushing them. At this point it becomes hard to resist the invitation that dualism extends to eliminativism, the view that mental phenomena do not exist at all.

Eliminativism may at first seem like a preposterous position. Like many extreme philosophical doctrines, however, it is worth taking seriously, both because it forces its opponents to produce illuminating arguments against it and because certain versions of it may actually turn out to be plausible for specific classes of mental phenomena.

The need for nontendentious evidence

One might be tempted to dismiss at least a blanket eliminativist view that denies the reality of any mental phenomenon by asking how any such theory could explain one’s own present conscious thoughts and experiences. But here it is crucial to keep in mind a principle that should be observed in any rational debate: in arguing against a position, one must not presuppose claims that the position explicitly denies. Otherwise, one is simply begging the question. Thus, it is no argument against a Newtonian account of planetary motion that it does not explain the fluttering of the angelic planet pushers’ wings, since precisely what the Newtonian account denies is that one needs to posit angels to explain planetary motion. Similarly, it is no argument against someone who denies mental phenomena that his view does not explain conscious experiences. “What conscious experiences?” the eliminativist might ask. What is needed in defending the existence of either angels or mental states is nontendentious data for the postulation in question.

This is, at first blush, a difficult challenge to meet. It is not obvious what nontendentious evidence for the existence of minds could consist of; indeed, their existence is actually presupposed by some of the evidence one might be tempted to cite, such as one’s own thoughts and other people’s deliberate actions. However, nontendentious evidence can be provided, and regularly is.

Consider standardized aptitude tests, such as the Scholastic Assessment Test (SAT) and the Graduate Record Examination (GRE), which are regularly administered to high school and college students in the United States. Here the standardization consists of the fact that both the question sheets and the answer sheets are prepared so as to be physically type-identical—i.e., the question sheets consist of identically printed marks on paper, and the answer sheets consist of identically printed rectangles that are supposed to be filled in with a graphite pencil, thus permitting a machine to score the test. Consider now the question sheets and the completed answer sheets that make up a single test that has been administered to millions of students at about the same time. The observable correlations between the printed marks on the question sheets and the graphite patterns on the answer sheets will be, from any scientific point of view, staggering. Overwhelmingly, students will have produced approximately the same graphite patterns in response to the same printed marks. Of course, the correlations will not be perfect—in fact, the answer sheets are supposed to differ from each other in ways that indicate likely differences in the students’ academic abilities. Still, the correlations will be well above any reasonable standard of statistical significance. The problem for the eliminativist is how to explain these standardized regularities without appealing to putative facts about the test takers’ mental lives—i.e., to facts about their thoughts, desires, and reasoning abilities.

Here it is important to remember that, in general, what science is in the business of explaining is not this or that particular event (or event token) but the regularities that obtain between different kinds of events (or event types). Although the fact that every token physical movement of every test taker is explainable in principle by physical theories, this is not in itself a guarantee that the types of events that appear in these correlations can also be so explained. In the case of standardized regularities, it is hard to think of any purely physical explanation that stands a chance.